1. Field of the Invention
The present invention relates to a solid-state image pickup device suitable for increasing the number of pixels and which is suitable for being miniaturized and a module type solid-state image pickup device.
2. Description of the Related Art
It is customary that a solid-state image pickup device includes a plurality of pixels, each pixel including a photoelectric-converting portion (photodiode PD, sensor portion) for photoelectrically converting incident light into signal electric charges and accumulating the thus photoelectrically-converted signal electric charges, an accumulation region (floating diffusion region FD) for converting the thus accumulated signal electric charges into a voltage and a transfer gate for transferring the signal electric charges accumulated in this photoelectric-converting portion to the accumulation region (floating diffusion region FD), arrayed in an XY matrix fashion (that is, in a two-dimensional fashion).
As the number of pixels in this solid-state image pickup device is increased more and this solid-state image pickup device is miniaturized more, the size of the unit cell is reduced more. Concurrently therewith, a ratio with which an area of this pixel is occupied by a transistor is increased and the area of the photodiode serving as the photoelectric-converting portion is decreased, thereby resulting in an amount of saturation electric charges being decreased. Since image quality is lowered as the amount of saturation electric charges of the photodiode serving as the photoelectric-converting portion is decreased, the decrease of the amount of saturation electric charges becomes a serious problem in this solid-state image pickup device.
FIG. 1 of the accompanying drawings is a schematic plan view showing an example of an arrangement of one pixel of a solid-state image pickup device according to the related art. In FIG. 1, reference numeral 1 denotes a photodiode (PD) comprising a photoelectric-converting portion, reference numeral 2 denotes a transfer gate and reference numeral 3 denotes a floating diffusion (FD) region.
FIG. 2A is a cross-sectional view taken along the line II-II in FIG. 1. In FIG. 2A, reference numeral 4 denotes a p-type semiconductor substrate. As shown in FIG. 2A, an n-type region 1a is formed on this p-type semiconductor substrate 4 to form the photodiode (PD) 1.
Also, an n+ type region 3a is formed on this p-type semiconductor substrate 4 to form the floating diffusion (FD) region 3 and a transfer gate 2 is formed on the p-type semiconductor substrate 4 between the n-type region 1a of the photodiode 1 and the n+ type region 3a of the floating diffusion (FD) region 3 through an insulating layer 5 formed of an SiO2 film.
Also, in FIG. 2A, reference numeral 6 denotes a reset drain formed of an n+ type region 6a. A reset gate 7 is formed on the p-type semiconductor substrate 4 between the n+ type region 3a of the floating diffusion (FD) region 3 and the n+ type region 6a of the reset drain 6 through the insulating layer 5.
In this case, the n-type region 1a, the transfer gate 2 and the n+ type region 3a constitute a read MOS (metal-oxide semiconductor) transistor 12 for reading pixels to the floating diffusion (FD) region 3. The n+ type region 3a, the reset gate 7 and the n+ type region 6 constitute a reset MOS transistor 14 for resetting signal electric charges of the floating diffusion (FD) region 3.
FIG. 2B shows a potential distribution obtained when pixels are accumulated in the solid-state image pickup device shown in FIG. 2A. FIG. 2C shows a potential distribution obtained when a read signal is supplied to the transfer gate. FIG. 2D shows a potential distribution obtained after the read signal was read out from the transfer gate 2.
There has hitherto been proposed a method of increasing the amount of saturation electric charges of the photodiode (PD) 1. As shown in FIG. 3A, the n-type region 1a is changed to an n+ type region 1b by increasing the dose of ions of n-type impurities implanted into this photodiode (PD) 1, whereby the potential depth of the photodiode (PD) 1 is increased as shown in FIG. 3B. Alternatively, as shown in the cited patent reference 1, the amount of saturation electric charges is increased by flattening the potential shape of the photodiode. [Cited patent reference 1]: Official gazette of Japanese laid-open patent application No. 2000-164849.
However, when the number of pixels of this solid-state image pickup device is increased and this solid-state image pickup device is miniaturized, the unit size of one pixel is reduced and there is a limit to increasing the amount of saturation electric charges of this photodiode (PD) 1. If the area of the floating diffusion (FD) region 3 is not increased by increasing the amount of saturation electric charges more than this limit, then when the read signal is supplied to the transfer gate 2, as shown in FIGS. 3C and 3D, all electric charges accumulated in the photodiode (PD) 1 cannot be transferred to the floating diffusion (FD) region 3 and the electric charges are left in the photodiode (PD) 1. There is then a disadvantage that a residual image is generated.